1. Technical Field
The disclosure relates generally to a method for manufacturing improved Fresnel lens.
2. Description of Related Art
It is known to use, in order to cut down the weight and cost of wide aperture lenses, so-called Fresnel lenses are developed, in which each Fresnel lens comprises a central lenticular portion surrounded by concentrically arranged prismatic rings. In actual practice, Fresnel lenses of this general type are usually provided with a first smooth face, of either flat or conical or frustum or spherical configuration, and a second face formed with a series of saw-like teeth encircling a centrally located core lens. These teeth extend across their related Fresnel lens face as a set of concentric rings comprising a refractive portion, which has an inclination arranged to vary from one tooth to the next, or between concentric rings, and a connection slope or flank in the form of a steeply inclined front arranged to interconnect the refractive portion of one tooth with the refractive portion of an adjacent tooth.
The optical quality of Fresnel lenses essentially depends on the shape of said teeth, and in particular of said connection slopes or flanks which performing no optical functions, but merely that of interconnecting two consecutive refractive portions together are required to interfere as little as possible with the light path. In ideal technical conditions, said connection slopes or steep fronts of the teeth should extend substantially parallel to the direction of the light beam which is being transmitted through the lens, so as to induce the least possible disturbance. However, this optimum situation can not be achieved in actual practice, even when the above problem is overcome.
The Fresnel lenses are conventionally formed from a plastic by hot-press molding technique. The plastic for forming Fresnel lenses is obtained by mixing two or more solutions together. In fact, when the two or more solutions are mixed together, or when the mixture is poured into a lower mold 100 of the related mold assembly (see FIG. 2 of the present disclosure), or when the upper mold of the related mold assembly is removed from the mixture, a mass of air bubbles will be formed in the mixture for forming the Fresnel lens. FIG. 2 shows that air bubbles 110 are formed in the mixture when pouring the mixture onto the lower mold 100 of the related mold assembly. The air bubbles severely interfere with said light path. Therefore, the optical performance of the Fresnel lens is still adversely affected.
What is need therefore is a method for manufacturing Fresnel lenses which can overcome the above-mentioned disadvantages.